An X-ray computed tomography (CT) apparatus is intended to reconstruct a tomographic image of an object using projection data from a plurality of angles that is obtained by rotating an X-ray source, which emits X-rays to an object, and an X-ray detector, which detects the amount of X-rays transmitted through the object as projection data, around the object, and display the reconstructed tomographic image. An image displayed in the X-ray CT apparatus is intended to depict, for example, the shape of an organ of the object, and is used for diagnostic scanning.
As a general trend, in the X-ray CT apparatus, there is a trade-off between the exposure dose and the image quality. That is, a tendency that the amount of image noise in the reconstructed image increases and the visibility of a lesion or the like becomes worse when reducing the dose at the time of scanning in order to reduce the exposure dose can be seen. Currently, in the X-ray CT field, there is an “X-ray automatic exposure mechanism” that appropriately controls the exposure dose based on the image quality indicator. As the image quality indicator, a standard deviation (hereinafter, referred to as SD) of image noise or a contrast-to-noise ratio (hereinafter, referred to as CNR) is used. In the following description, a mode using an X-ray automatic exposure mechanism based on image noise that the operator desires (hereinafter, an image noise target value) is referred to as an SD mode, and a mode using an X-ray automatic exposure mechanism based on the CNR that the operator desires (hereinafter, a CNR target value) is referred to as a CNR mode.